Many imaging systems operate during both day and night. For example, security imaging systems often capture surveillance photos or surveillance video twenty four hours a day. As another example, automobile rearview imaging systems typically must be capable of capturing video during both day and night.
During the day, sunlight is normally sufficient to illuminate a scene for imaging. However, for nighttime imaging, an artificial light source is usually required to sufficiently illuminate a scene. Although the artificial light source can be a visible light source, the artificial light source is commonly an infrared light source. Accordingly, many imaging systems that operate during both night and day must be capable of capturing images under both visible light and infrared light scene illumination.
It is common, though, for an image scene to be illuminated by both visible and infrared light. For example, sunlight contains infrared light, as well as visible light. As another example, a scene illuminated at night primarily by an infrared artificial light source may also be partially illuminated by visible light from a nearby stray light source. Thus, an image sensor that is sensitive to both visible and infrared light will often generate image data from both visible and infrared light. It is generally undesirable, however, to mix visible light image data and infrared light image data because they may interfere with each other.
One conventional approach to preventing simultaneous generation of image data from both visible and infrared light is to filter light incident on an image sensor according to expected or desired illumination. In particular, during the day, or during times when visible light illumination is anticipated, an infrared blocking filter is mechanically switched into the image sensor's optical path, thereby blocking infrared light from reaching the image sensor. During the night, or during times when infrared light illumination is anticipated, the infrared blocking filter is removed and a visible light blocking filter is switched into the optical path, thereby preventing visible light from reaching the image sensor. Although such filtering techniques may provide satisfactory results, they generally require moving parts which are typically expensive and failure prone.